A number of machines have been manufactured for heat forming bags or pouches from a continuous roll of thermoformable plastic film material. For example, U.S. Pat. No. 4,229,244 is directed to a manual bag sealer having a lift bar. The heat sealing apparatus includes a fixed upper clamping jaw that has a continuously energized resistance-type heater. The heat sealing apparatus also includes a lower clamping jaw that is pivotally supported for movement about a substantially horizontal axis to upwardly engage the upper jaw and clamp a mouth of a filled, preformed bag between the upper and lower jaws in order to seal the bag. However, the present bag sealing apparatus is a manual device for sealing a filled bag. Accordingly, such construction will not work well with an automated thermoforming line for packaging stacks of accumulated articles in individual bags.